Matter Revision Notes

Defining Specific Heat Capacity (c)

Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1 °C (or 1 K). Its units are J/kg°C or J/kgK. Different substances have different specific heat capacities; a high value means it takes more energy to change its temperature, and a low value means less. For example, water has a high specific heat capacity, which is why it warms up slowly but also cools down slowly.

Defining Specific Latent Heat (L)

Specific latent heat is the energy required to change the state of 1 kg of a substance without changing its temperature. During a change of state (like melting, freezing, boiling, or condensing), all applied energy is used to break or form intermolecular bonds, not to increase particle kinetic energy (temperature). Its units are J/kg. There are two types: specific latent heat of fusion (for melting/freezing) and specific latent heat of vaporisation (for boiling/condensing).

Key Differences and Applications

The crucial difference is that specific heat capacity relates to temperature changes within a single state (e.g., heating water from 20°C to 80°C), while specific latent heat relates to state changes at a constant temperature (e.g., melting ice at 0°C or boiling water at 100°C). We use specific heat capacity to calculate energy needed to change temperature (Q = mcΔT) and specific latent heat to calculate energy needed for state change (Q = mL). These concepts are fundamental to understanding thermal energy transfer in various applications, from cooking to weather patterns.

PAG P5: Investigating Specific Heat Capacity

PAG P5 involves measuring specific heat capacity, for example, of a metal block. A typical method includes: insulating the block, inserting an immersion heater and thermometer. Measure mass (m), initial temperature (T1), then switch on the heater for a set time (t) with a known power (P) or current (I) and voltage (V). Measure final temperature (T2). Energy supplied (Q) = P x t. Energy absorbed by block = m x c x (T2 - T1). Equating these allows calculation of 'c'. This assumes no heat loss to surroundings, which is a potential source of error. The independent variable is the energy supplied, the dependent variable is the temperature change, and control variables include the mass of the block and the immersion heater power.

Internal Energy and Temperature (Higher)

The internal energy of a gas is the total energy stored by its particles, comprising both their kinetic and potential energies. For gases, especially ideal gases, the potential energy due to intermolecular forces is often negligible, so internal energy is primarily the total kinetic energy of the particles. Temperature is directly proportional to the average kinetic energy of these particles. Therefore, increasing the internal energy of a gas, by increasing the kinetic energy of its particles, directly results in an increase in its temperature.

Doing Work on a Gas (Higher)

Work is done on a gas when an external force acts to compress it, such as pushing a piston into a cylinder filled with gas. This process involves energy transfer from the external force to the gas. When the piston moves inward, it collides with the gas particles, transferring kinetic energy to them. These particles, now moving faster, then collide with other gas particles, distributing this increased kinetic energy throughout the gas.

Mechanism of Temperature Increase (Higher)

The energy transferred to the gas particles during compression increases their individual kinetic energies. As the average kinetic energy of the gas particles increases, the overall internal energy of the gas rises. Since temperature is a direct measure of the average kinetic energy of the particles, this increase in average kinetic energy is observed as a rise in the gas's temperature. A practical example is the heating of a bicycle pump when air is compressed, or the high temperatures reached during the compression stroke in a diesel engine.

Understanding Changes of State

Changes of state include melting (solid to liquid), freezing (liquid to solid), evaporating (liquid to gas), condensing (gas to liquid), and sublimating (solid to gas). These are all physical changes, meaning the chemical identity of the substance does not change. For example, ice, liquid water, and steam are all H₂O molecules; only their arrangement and energy differ.

The Principle of Conservation of Mass

The Law of Conservation of Mass states that mass cannot be created or destroyed. When a substance changes state, the total number of atoms and molecules remains the same. The particles simply move further apart or closer together, or change their arrangement, but their individual masses are unchanged, leading to an overall constant total mass.

Mass in Closed vs. Open Systems

In a closed system, where no matter can enter or leave, the mass before and after a change of state will be exactly the same. For instance, if you melt ice in a sealed container, the total mass of the container and its contents will not change. In an open system, such as water evaporating from an open pan, it might seem like mass is lost because the water vapour disperses into the surroundings. However, the mass is not destroyed; it has merely moved out of the measured system. If you could collect all the dispersed water vapour, you would find the original mass is still conserved.

Practical Demonstration: Conserving Mass

A simple experiment to demonstrate conservation of mass involves placing ice cubes in a sealed plastic bag or conical flask with a stopper, and measuring the initial total mass. Allow the ice to melt completely into water at room temperature. After melting, measure the total mass again. You should observe that the mass remains constant. This confirms that melting, a change of state, does not alter the total mass of the substance.

Molecular Motion in Gases

Gases consist of particles (atoms or molecules) that are in continuous, random motion. These particles move at high speeds and frequently collide with each other and with the walls of their container. The average kinetic energy of these particles is directly related to the absolute temperature of the gas. The more energetic the particles, the faster they move and the more kinetic energy they possess.

Pressure and Molecular Collisions

The pressure exerted by a gas is a result of the total force produced by the constant collisions of the gas particles with the interior walls of the container. Each time a particle strikes a wall, it exerts a tiny force. The sum of these innumerable forces over the entire surface area of the container walls gives rise to the measurable pressure. Pressure is defined as force per unit area.

Temperature's Effect on Molecular Motion

Increasing the temperature of a gas causes its particles to move faster, on average. This increased speed means that the molecules have higher kinetic energy. As a result, they collide with the container walls more frequently, and each collision exerts a greater force. This combined effect leads to an increase in the overall pressure exerted by the gas, assuming the volume remains constant.

Temperature's Effect on Gas Pressure

Conversely, decreasing the temperature of a gas reduces the average speed of its particles. They possess less kinetic energy, leading to less frequent collisions with the container walls and each collision exerting a smaller force. This reduction in the overall force exerted on the walls per unit area results in a decrease in the gas's pressure, again assuming the volume is kept constant.